X-ray conversion of ultra-short laser pulses on a solid sample: Role of electron waves excited in the pre-plasma

Flat silicon samples were irradiated with 40 fs, 800nm laser pulses at an intensity at the best focus of 2.10(18) Wcm(-2), in the presence of a pre-plasma on the sample surface. X-ray emission in the spectral range from 2 to 30 keV was detected inside and outside the plane of incidence, while varying pre-plasma scale length, laser intensity, and polarization. The simultaneous detection of 2 omega and 3 omega/2 emission allowed the contributions to the X-ray yield to be identified as originating from laser interaction with either the near-critical density (n(c)) region or with the n(c)/4 region. In the presence of a moderate pre-plasma, our measurements reveal that, provided the pre-plasma reaches a scale-length of a few laser wavelengths, X-ray emission is dominated by the contribution from the interaction with the under dense plasma, where electron plasma waves can grow, via laser stimulated instabilities, and, in turn, accelerate free electrons to high energies. This mechanism leads also to a clear anisotropy in the angular distribution of the X-ray emission. Our findings can lead to an enhancement of the conversion efficiency of ultra short laser pulses into X-rays. (C) 2014 AIP Publishing LLC.